1. Field of the Invention
The present invention relates to DC/DC converters with a high step-down ratio, and particularly to technologies for downsizing the same.
2. Description of the Related Art
FIG. 1 is a circuit configuration diagram of a conventional DC/DC converter. This DC/DC converter is described in JP2000-308337 A. The DC/DC converter shown in FIG. 1 is a converter for large output current and low output voltage (a converter for a CPU, for example). In order to improve the response and allow a large electric current to flow, two converters are connected in parallel in the DC/DC converter, the converters operating with a 180° phase shift.
A diode D1 is connected to both ends of a DC power supply Vdc1 through a switch Q1 comprised of a MOSFET or the like. A diode D2 is connected to both ends of the DC power supply Vdc1 through a switch Q2 comprised of a MOSFET or the like. A series circuit including a reactor L1 and a smoothing capacitor C1 is connected to both ends of the diode D1. A series circuit including a reactor L2 and the smoothing capacitor C1 is connected to both ends of the diode D2. A load RL is connected across the smoothing capacitor C1.
The switch Q1, the diode D1, and the reactor L1 constitute a first converter. The switch Q2, the diode D2, and the reactor L2 constitute a second converter.
A control circuit 100 causes the high-frequency switching operation of the switches Q1 and Q2 with a 180° phase shift (half a cycle).
Next, an operation of the conventional DC/DC converter thus configured will be described with reference to a timing chart of signals shown in FIG. 2.
At a time t30, once the switch Q1 is turned on by a Q1 control signal Q1g from the control circuit 100, an electric current flows through a path from a positive side of the DC power supply Vdc1, to the switch Q1, to the reactor L1, to the smoothing capacitor C1, and then to a negative side of the DC power supply Vdc1. Accordingly, an electric current Q1i in the switch Q1 linearly increases. At the same time, an electric current L1i in the reactor L1 also linearly increases.
At a time t31, once the switch Q1 is turned off by the Q1 control signal Q1g from the control circuit 100, the electric current Q1i in the switch Q1 rapidly becomes zero. At this time, the energy stored in the reactor L1 causes an electric current to flow through a path from the reactor L1, to the smoothing capacitor C1, to the diode D1, and then to the reactor L1. Accordingly, the energy is supplied to the load RL via the smoothing capacitor C1. The electric current L1i in the reactor L1 also decreases from its peak value with a gradient corresponding to the difference value between the input voltage and the output voltage.
At a time t32, once the switch Q2 is turned on by a Q2 control signal Q2g from the control circuit 100, an electric current Q2i in the switch Q2 linearly increases. At the same time, an electric current L2i in the reactor L2 also linearly increases.
At a time t33, once the switch Q2 is turned off by the Q2 control signal Q2g from the control circuit 100, the electric current Q2i in the switch Q2 rapidly becomes zero. At this time, the energy stored in the reactor L2 causes an electric current to flow through a path from the reactor L2, to the smoothing capacitor C1, to the diode D2, and then to the reactor L2. Accordingly, the energy is supplied to the load RL via the smoothing capacitor C1. The electric current L2i in the reactor L2 also decreases from its peak value with a gradient corresponding to the difference value between the input voltage and the output voltage. The operation carried out at a time t34 is similar to that carried out at the time t30.
However, the DC/DC converter shown in FIG. 1 requires two reactors L1 and L2. In addition, if a perfect symmetry between the two converters inclusive of the circuit wiring cannot be maintained, the electric currents in these converters cannot be balanced, and the uneven loss will be caused. Moreover, a correction circuit or the like, which carries out correction in order that the electric currents in the converters are balanced, becomes necessary, which has resulted in a disadvantage that the circuit becomes complicated.